Re: Addr: Re: BGP-4 - revised I-D

[Curtis Villamizar <[e-mail]>]

In message <[e-mail]>om>, Tony Li writes:
> 
>    > - group the routes by neighboring AS
>    > - within each group, select the routes with the best MED
>    > 
>    > which is also what I think is the intended interpretation.  Alternate
>    > interpretations include:
>    > 
>    > i)
>    > - group the routes by neighboring AS
>    > - within each group, select _a_ route with the best MED
> 
>    Interpretation i) is the same as the first.  
> 
> No, it's not.  If you follow i) verbatim, then you must select one and only
> one route.  As we have not specified which one, it's presumably left up to
> the implementor.  You and I both know that this is disaster.  ;-)
> 
>    If there are two routes
>    with the same AS and the same MED, the next criteria is IGP cost or
>    advertising router IP address.  Whether you bring one or both into the
>    next comparison, you end up with the same decision whether you
>    consider MED and they tie in the first comparison or you ignore MED in
>    the second comparison.
> 
> Did you really mean to say this?  Suppose I have routes A, B, C, ... and
> that A & B are the same MED.  Without loss of generality, suppose that A
> has a better IGP metric.  
> 
> If we bring A into the next comparison, then it will be selected.  If we do
> NOT bring A into the next comparison, then B may be selected.  Loop.
> 
> Tony


Tony,

A & B are the same MED.  A has the lower IGP cost.  Bring A into the
next level of selection.

The algorithm is very simple.

  get a new route in a BGP update

  if the oute passes policy filter criteria, compare to the current
  best route for the AS using all criteria - local-pref, med, igp,
  advertiser IP address

  if the best route for the AS changed, compare it to the overall best
  route using all the criteria except MED - local-pref, igp,
  advertiser IP address

  if the new route is better you have a new best route.

Simple to explain.  Simple to implement.  No loop.

Curtis